According to conventional circuit designs, a circuit of source follower is generally equipped in the front end of a programmable gain amplifier (PGA). For a programmable gain amplifier, an output resistance supplied by a front circuit of a circuit with a source follower is lower than an output resistance supplied by a front circuit of a circuit without a source follower. In an implementation, metal-oxide-semiconductor field-effect transistors (MOSFETs) may be used as a buffer and converted impedance in a source follower. However, since components in the source follower may deteriorate due to the effects of process, voltage and temperature (PVT), the source follower outputs unexpected voltage and current. Therefore, the overall efficiency of the circuit may be degraded.